This invention pertains to the method and means for firmly supporting photographic materials in accurate registration during contact exposure, and more particularly to a passive hold-down apparatus and method in which the region beneath an impervious membrane is selectively evacuated in a manner that develops a hold-down force which does not significantly alter registration of the photographic materials.
Contact exposure of photographic copy and master sheets requires accurate registration between such sheets during exposure. Certain known schemes rely upon the sheets being pressed together and against an exposure plate by various mechanisms in order to maintain the sheets in accurate registration during exposure through the exposure plate. One known mechanism depends upon a flexible, weighted blanket being rolled out over the photographic sheets to hold them in place on the exposure plate. Still other known mechanisms rely upon air pressure or resilient means to force the photographic sheets together and against the exposure plate during exposure. Mechanisms of these types are disclosed in the literature (see, for example, U.S. Pat. Nos. 4,029,404 and 4,551,016 and 4,674,868 and 4,707,125).
One common disadvantage associated with mechanisms of these types is that air pockets may remain at random locations between the photographic master and copy sheets and between the exposure plate and sheets that may produce distorted photographic images due to non-contacting reproduction where the air pockets are located. Surface-textured vacuum blankets have been used to facilitate removal of air (see, for example, U.S. Pat. No. 4,705,392). Excessive pressure applied to the photographic sheets to assure that no air pockets remain unnecessarily increases the weight, strength and costs of the exposure plate and associated supporting structures, in order to withstand the pressing forces, and may contribute to distortion of the sheets which may be forced to `flow` and reform around an air pocket. Even dimensionally stable photographic sheets may stretch to some extent when pulled unevenly, and may be shifted out of position and registration when pressure is applied and covering blanket shifts or stretches. In addition, excessive pressure applied to photographic sheets, particularly of plastic materials, introduces stresses which may form Newton rings in the reproduction, especially under exposure to collimated light.
These disadvantages have been resolved to some degree by use of `slip-sheet` or extra layer of film such as Mylar (Trademark of DuPont Co.) between the material to be exposed and the covering vacuum blanket. This film may be moved or pulled slightly by the covering blanket during development of hold-down force upon evacuation of air beneath the blanket without moving or pulling the material to be exposed out of position. However, this scheme involves additional set-up time, and the `slip-sheet` must be stored while not in use, and must be kept clean to avoid transferring dust onto the material being exposed, and typically must be replaced at frequent intervals.